The present disclosure relates to polymeric soil reinforcing and geotechnical articles. Strips, sheets, profiles, and cellular confinement systems are made from polymer compositions, such as a multiphase polymeric material, which are optimized for use in subzero temperatures.
Polymeric or plastic soil reinforcing articles, especially cellular confinement systems (CCSs), are used to increase the load bearing capacity, stability and erosion resistance of geotechnical materials such as soil, rock, stone, peat, clay, sand, concrete, aggregate and earth materials which are supported by said CCSs.
CCSs comprise a plurality of high density polyethylene (HDPE) or medium density polyethylene (MDPE) strips in a characteristic honeycomb-like three-dimensional structure. The strips are welded to each other at discrete locations to achieve this structure. Geotechnical materials can be reinforced and stabilized within or by CCSs. The geotechnical material that is stabilized and reinforced by the said CCS is referred to hereinafter as geotechnical reinforced material (GRM). The surfaces of the CCS may be embossed to increase friction with the GRM and decrease relative movement between the CCS and the GRM.
The CCS strengthens the GRM by increasing its shear strength and stiffness as a result of the hoop strength of the cell walls, the passive resistance of adjacent cells, and friction between the CCS and GRM. Under load, the CCS generates powerful lateral confinement forces and soil-cell wall friction. These mechanisms create a bridging structure with high flexural strength and stiffness. The bridging action improves the long-term load-deformation performance of common granular fill materials and allows dramatic reductions of up to 50% in the thickness and weight of structural support elements. CCSs may be used in load support applications such as road base stabilization, intermodal yards, under railroad tracks to stabilize track ballast, retaining walls, to protect GRM or vegetation, and on slopes and channels.
The term “HDPE” refers hereinafter to a polyethylene characterized by density of greater than 0.940 g/cm3. The term medium density polyethylene (MDPE) refers to a polyethylene characterized by density of greater than 0.925 g/cm3 to 0.940 g/cm3. The term linear low density polyethylene (LLDPE) refers to a polyethylene characterized by density of 0.91 to 0.925 g/cm3.
Current commercially available CCSs are generally made solely from HDPE. CCS cell walls made from HDPE are stiff in the vertical direction, maintain some flexibility in the horizontal direction, are dimensionally stable, resist creep relatively well at temperatures from minus 10 to +40° C., and have sufficient stiffness when the cells are empty so the CCS can be applied. GRM is then provided by generally dumping the GRM onto the CCS, then packing the cells within the CCS. If the CCS wall is too flexible, it will collapse during installation in the field, for example when humans walk over the CCS before it is filled with GRM or during the filling and condensing of GRM in the CCS cells.
However, HDPE is relatively rigid; it has a 1% secant flexural modulus according to ASTM D790 of about 950 megapascals (MPa). This rigidity makes the web possible to handle and usable in field operations at ambient conditions (20 to 30° C.). However, installation becomes difficult and sometimes impossible at subzero temperatures, especially temperatures below minus 10° C. HDPE also has high tendency to creep at temperatures of about +40° C. and over.
HDPE is also brittle at low temperatures (i.e. below minus 15° C.) that are typical of places on the globe north of 42 degrees north latitude and south of 42 degrees south latitude (i.e., not near the equator) during certain times of the year, usually autumn and winter. Such low temperatures are also encountered at elevated areas (i.e., about 1000 m above sea level and higher) all over the globe. These areas at which low temperatures are typical are referred to hereinafter as “cold areas”. HDPE and MDPE also have poor puncture resistance at cold temperatures. At temperatures lower than minus 10° C., these polymers are no longer tough and ductile, but fragile and brittle.
Two major factors affecting the durability of the CCS are the creep resistance of the plastic material making up the CCS wall and the friction between the cell walls and the GRM. Creep of the CCS wall causes loosening of the friction and loss of structural functionality of the CCS-GRM composite structure. HDPE and other polyolefins fail to resist creep, especially at temperatures higher than about 35-40° C.
The mechanical properties of filled CCSs are a composite phenomenon wherein stiffness and rigidness come from the compacted infill (GRM) being compressed and densified along the plastic CCS cell walls. Friction between the GRM and the cell walls provide integrity, mechanical continuity, and dynamic load bearing. The GRM and the cell walls dynamically support each other and can survive a wide spectrum of loads, vibrations, thermal stresses, and erosion as long as this relationship is maintained. Anytime the load transfer between those two components is breached—due to cell wall creep, rupture or irreversible deformation—the filled CCS structure loses its integrity and cannot provide the required structural strength, dimensional stability and stiffness.
The mechanism of failure of CCSs made from HDPE at sub-zero temperatures is complex. The first step is the cooling of the GRM and CCS. Polyethylene has a high coefficient of thermal expansion (CTE)—about 150-200 ppm/° C. In other words, a 100 meter strip will shorten by about 15-20 centimeters when cooled from minus 15° C. to minus 25° C. However, the GRM generally has a CTE about 5-10 times lower. Because the GRM shrinks much less, stress is generated in the cell walls of the CCS. When the stress is applied for many days at temperatures lower than minus 15° C., the toughness of the HPDE or MDPE is insufficient and a brittle failure occurs. If the GRM is subjected to freezing of water, which expands the GRM, the stress is increased even more. Since crack growth resistance (toughness) of HDPE and MDPE is medium or even low relative to LLDPE or elastomers at those temperatures, the CCS breaks and loses its integrity. However, if LLDPE or elastomers are used as the matrix of the CCS, then the CCS severely creeps at temperatures greater than 40° C. Thus, the repeated cycles of heating, expansion of the CCS, resultant spreading or collapse of the GRM structure previously contained by the CCS, results in eventual failure or significant loss of function of the CCS.
This brittleness also impacts CCSs. In particular, this brittleness critically affects the weld points between the plurality of strips. The welds are relatively weak points; thus, any negative aspect of the polymers is magnified at the welds. In addition, CCSs are usually stabilized to the GRM or other infrastructure materials by anchors, tendons, and/or wedges. Because the connection points between the CCS and the anchors, tendons, and/or wedges have high loads concentrated in a small area, failure is most likely to happen at these stress concentration points, especially under extreme conditions such as subzero temperatures or temperatures higher than about 40° C.
HDPE also has relatively poor stress cracking resistance, medium to low tear and puncture resistance, and low crack growth resistance—especially at subzero temperatures. Cracks are initiated in geotechnical applications during application and installation, and during service, especially when dynamic loads are applied. Crack growth resistance is a temperature-dependent phenomenon, wherein as temperature decreases, the material becomes more brittle and less damage tolerant. Since the brittleness increases in a “quasi-exponential” fashion as temperature drops, reinforcing articles comprising HDPE as the major constituent are subjected to catastrophic failure at subzero (° C.) temperatures. Again, failure is more likely to happen at the weld points and at the contact points between the CCS and wedges, anchors, and tendons.
Stress is also generated at the welds between the strips making up the CCS. Stress can be applied from compression when GRM is dumped onto the CCS to fill the cells. GRM can also expand when it becomes wet or when water already in the GRM freezes in cold weather. In addition, GRM has a coefficient of thermal expansion (CTE) about 5-10 times lower than the HDPE used to make the strips. Thus, the HDPE will either expand more than or shrink less than the GRM contained in the cells; this causes stress at the welds as well.
More flexible polymers, such as linear low density polyethylene (LLDPE), are better than HDPE in subzero temperature conditions. However, they have very poor creep resistance at temperatures higher than ambient, and especially higher than 40° C. Such high temperatures are expected in arid and tropic areas, but are also reached in cold areas (e.g., during the summer). Another drawback of relatively flexible polymers (such as LLDPE) is that they lack the stiffness needed when the CCS cells are still empty and humans need to walk on it during installation or during filling and compaction of GRM. If the CCS wall is too flexible, it will collapse during installation in the field, especially during the filling and condensing of GRM in the CCS cells. They also tend to creep under load, so that the connection points to anchors, tendons, and/or wedges get loose over time in elevated temperatures. This creep undermines the integrity of the CCS.
The only current working solution for cold areas are special HDPE compositions that are characterized by bimodal chain distribution, wherein one type of chain is relatively stiff and the second type of chain is relatively flexible. These polymers are made in a reactor and thus very limited in composition flexibility. If a more rubbery phase is required, it cannot be made in a reactor. These special compositions also have a relatively higher cost—usually 20-30% more than regular HDPE. Despite the advantage of two kinds of polyethylene in one resin, these resins still creep at temperatures greater than 40° C., have a CTE higher than 150 ppm/° C., and have high viscosity.
U.S. Pat. No. 3,963,799 provides compositions of polyamide and polyolefin, adapted mostly for packaging industry and methods to form alloys (compatibilized blends) thereof. The compositions described in this patent are not applicable for structural geotechnical applications including CCSs, due to its inherent brittleness, especially at low temperatures, and lack of protection against humidity and UV light. This patent does not deal with either the difficulties in welding of the compositions, or the hydrolytic instability of the polyamide phase, which may be hydrolyzed in soil, especially acidic soils.
In U.S. Pat. No. 4,346,834, different types of polyethylenes are blended to overcome the brittleness of HDPE and the low puncture resistance of LDPE and LLDPE. However, LLDPE itself does not provide adequate flexibility at low temperatures. Also, because the molecular structures of HDPE or MDPE and a more flexible polyethylene like LLDPE are different, they are immiscible and require intensive mechanical energy and adequate residence time to provide balanced physical properties. Blending in standard manufacturing equipment does not provide the morphology that is required for long-term stability when the temperature of the exposed plastic can vary from minus 70° C. to +90° C. This is a problem especially in cold areas where during autumn and winter, the temperature of exposed plastics may drop below minus 40° C., but during summer, when direct sunlight is absorbed by the CCS surface, temperatures may exceed +90° C. (especially on dark-colored surfaces). A similar approach is described in U.S. Pat. No. 6,355,733. Other drawbacks related to this mixing of two polyethylenes for geotechnical articles are the still high CTE (higher than about 150 ppm/° C.), poor heat conductivity, high creep under loads provided during thawing of water in the GRM pores, limited chemical resistance to oils and fuels (for example oilfields in Alaska and Siberia), and difficulties in high throughput extrusion of film and strips due to the low melt flow index of LLDPE.
U.S. Pat. No. 4,564,658 provides compositions of polyester and linear low density polyethylene (LLDPE) only, and provides no compatibilizer, i.e., no agent to stabilize the dispersion of the two immiscible polymers. Consequently, in extrusion applications, for example extrusion of strips for geotechnical applications, flow of the melt is uneven (melt fracturing), and segregation between phases is observed. The compositions described in this patent are not applicable for structural geotechnical applications including CCSs, due to their flexibility and creep tendency. The patent also does not provide a solution for the protection of the blend from hydrolysis in soils and landfills, oils and hydrocarbons, and from the degradation induced by heat and UV light. Welding quality is not discussed. Another drawback is that LLDPE is not flexible enough and lacks the required toughness when it reaches temperatures lower than minus 40° C.
U.S. Pat. No. 5,280,066 provides compositions of polyester, polyolefin and a functionalized styrenic elastomer for improved impact resistance, especially for injection molding. The invention is limited only to polypropylene (PP) as the polyolefin fraction. PP is too rigid and lacks the flexibility at temperatures below about 0° C., a property that is mandatory in CCSs. The compatibilizer in this patent is styrene-based and thus has limited UV light resistance, limiting the composition lifetime to about 1 to 2 years. Polyester blends, especially when not specially stabilized against hydrolysis, may fail in soils, especially those having pH greater than 7, within a relatively short period of time. Welding quality is not discussed. Another drawback is that the blend is not flexible enough and lacks the required toughness when it reaches temperatures lower than minus 40° C.
Similar compositions are described in U.S. Pat. No. 6,649,698, for improved stress cracking of films, including geomembranes. The incorporation of the more amorphous LLDPE into the HDPE resin provides crack stop mechanisms, but no solution is provided for CCS systems where strength is crucial and creep must be minimized—especially at temperatures greater than 40° C. Moreover, no solution is provided for subzero temperatures such as temperatures below minus 15° C. or minus 40° C. Since CCSs are a composite structure comprising strips and weld lines, mixing two different polyethylenes may negatively affect welding strength and long-term durability as well. A major limiting factor in a simple mixing of two polymers is that the CTE remains high and even increases, so the advantage of better rupture resistance at cold temperatures is negatively balanced by the higher CTE. Another disadvantage of blending LDPE or LLDPE with HDPE is that inferior weld strength results. It is also not straightforward to disperse the relatively viscous LLDPE in an HDPE matrix, especially by means of conventional extrusion equipment.
U.S. Pat. No. 6,875,520 provides compositions of polyamide block copolymer and a very flexible polyolefin. This invention may be useful for geomembranes but not for structural geotechnical applications including CCS. The high flexibility that is an advantage in geomembranes becomes a drawback in CCS: when load is applied on CCS supporting GRM, the composite structure of the two components interacts with the load as an integrated system. The CCS transfers the load from cell to cell by friction with the GRM which provides rigidity and stiffness. If the CCS is too flexible, the load induces a deformation of the CCS until friction with the GRM is lowered. At that specific state, the integrated system is irreversibly damaged and can no longer provide the required durability, stiffness and stabilization to the GRM. The patent does not provide a solution to the hydrolysis of the composition in soils and landfills, or when exposed to concrete or other media characterized by pH of greater than 7. UV and heat stability are not discussed or provided. The flexible blend has a CTE greater than 150 ppm/° C. and also does not provide sufficient toughness at temperatures lower than minus 40° C.
There is still a need for a geotechnical article, especially a CCS, that has excellent creep resistance, including at temperatures of about 40° C., a lower CTE, improved tear resistance, and high crack growth resistance at temperatures ranging from about minus 70° C. to about +90° C., maintains enough flexibility to enable installation and GRM filling at temperatures as low as minus 40° C., provides improved welding quality and strength compared to HDPE-based CCSs, especially under continuous loads at temperatures below minus 15° C., and provides improved resistance against UV and heat degradation. Such a CCS would be useful in cold areas of the earth.